Color television devices of the projection type for projecting video images on a screen in front of the device are provided with three video display devices for R (red), G (green) and B (blue). These display devices are arranged as directed toward the screen in front of the devices.
Each of the video display devices comprises a picture tube 2 and a deflection yoke device 10 as shown in FIG. 6. The picture tube 2 comprises a cone portion 21, a neck portion 22 projecting from a small-diameter end of the cone portion 21, and a face plate 23 formed at a large-diameter end of the cone portion 21, these components being joined into an integral assembly. The neck portion 22 has an electron gun 3 housed therein. The deflection yoke device 10 extends from part of the cone portion 21 of the tube 2 to the neck portion 22 thereof, as provided around these portions. In the drawings to be referred to hereinafter, the horizontal direction and the vertical direction of the picture tube 2 are represented by the X-axis and the Y-axis, respectively, and the direction toward which an electron beam is emitted from the electron gun 3 is represented by the Z-axis.
With reference to FIG. 7, the deflection yoke device 10 comprises a main yoke portion 4 for deflecting the electron beam emitted by the electron gun 3, and a convergence yoke portion 50 for adjusting the convergence of the electron beam. The convergence yoke portion 50 is positioned to the rear of the main yoke portion 4 in proximity thereto. Incidentally, FIG. 7 includes a side elevation partly broken away and showing the upper half of the deflection yoke device 10 and a view in vertical section and showing the lower half of the device 10.
The main yoke portion 4 comprises a main yoke horizontal coil 6 provided along the inner peripheral surface of a conical bobbin 80, a core 8 provided around the outer peripheral surface of the bobbin 80, and a main yoke vertical coil 7 wound around the core 8. By passing current through the main yoke horizontal coil 6, a horizontal magnetic field is produced inside the picture tube 2. A vertical magnetic field is set up inside the picture tube 2 by passing current through the main yoke vertical coil 7. The electron beam emitted by the electron gun 3 is deflected horizontally and vertically by these magnetic fields.
When a periodic sawtooth current is passed through the main yoke horizontal coil 6 and the main yoke vertical coil 7, the electron beam scans the face plate 23 of the picture tube 2 in a horizontal direction and a vertical direction, thereby producing images on the face plate 23. R, G, B images produced by the three video display devices are projected as enlarged on the screen in front, and the images are superimposed to display color images on the screen. In digital color television devices of recent years, a sawtooth current having a frequency of 31.5 KHz is used for horizontal scanning, and like current with a frequency of 60.0 Hz for vertical scanning.
With reference to FIG. 9, the convergence yoke portion 50 comprises an annular convergence core 53, a convergence horizontal coil 51 wound around the core 53, and a convergence vertical coil 52 similarly wound. The convergence horizontal coil 51 is wound around both of left and right two regions of the core 53 which intersect the X-axis and has opposite ends connected to a horizontal convergence correction circuit 11. The convergence vertical coil 52 is wound around both of upper and lower two regions of the core 53 which intersect the Y-axis and has opposite ends connected to a vertical convergence correction circuit 12.
When current is passed through the convergence horizontal coil 51 in a direction indicated by arrows in the drawing, a magnetic field is produced in the interior space S0 of the picture tube 2, the magnetic field pointing from above downward as indicated by solid-line arrows in FIG. 9. An electron beam 9 emanating from the electron gun 3 toward the viewer of the drawing perpendicular to the plane of the drawing is deflected by the magnetic field toward the right in the drawing. Further when current is passed through the convergence vertical coil 52 in a direction indicated by arrows in the drawing, a magnetic field pointing from the right toward the left is produced in the upper half of the interior space S0 and a magnetic field pointing from the left toward the right is produced in the lower half thereof as indicated by broken-line arrows in the drawing. The electron beam 9 is deflected by the magnetic fields downward in the drawing.
Accordingly, RGB three electron beams, even if deflected in any direction, can be converged toward one point on the face plate of the picture tube by adjusting the currents to be passed through the convergence horizontal coils 51 and the convergence vertical coils 52 of the R, G, B video display devices, whereby RGB three images can be perfectly superimposed on the screen.
When a current having a horizontal scanning frequency is passed through the main yoke horizontal coil 6 shown in FIG. 8, a fluctuation magnetic field m is set up around the base portion 60 of the coil 6 in a direction perpendicular to the flow of current. With reference to FIG. 7 showing the deflection yoke device 10, the convergence yoke portion 50 is positioned close to the base portion 60 of the main yoke horizontal coil 6, so that the fluctuation magnetic field m produced by the base portion 60 ingresses into the convergence yoke portion 50 on leakage, generating induction voltage in the convergence horizontal coil 51 due to the leakage field.
FIG. 10 shows the result obtained by measuring variations with time in the induction voltage produced in the convergence horizontal coil 51 when a sawtooth current having a horizontal scanning frequency of 31.5 KHz is passed through the main yoke horizontal coil 6, with a sawtooth current having a vertical scanning frequency of 60.0 Hz passed through the main yoke vertical coil 7. The graph reveals a marked rise in the voltage of the convergence horizontal coil 51 in horizontal scanning cycles of 31.75 μS. Presumably, this indicates that leakage of the fluctuation magnetic field m from the main yoke horizontal coil 6 produces induction voltage in the convergence horizontal coil 51. With actual devices, the fluctuation magnetic field of the main yoke portion periodically produces a voltage of at least 10 V in the convergence horizontal coil.
Thus with the deflection yoke device 10 shown in FIG. 7, the induction voltage produced in the convergence horizontal coil 51 of the convergence yoke portion 50 poses the problem of presenting difficulty in adjusting the convergence by the horizontal coil 51. This problem may presumably be solved by providing a circuit for removing the fluctuation magnetic field from the main yoke portion 4, whereas this not only makes complex the circuit to be connected to the deflection yoke device 10 but also entails the problem of necessitating expensive pressure-resistant circuit components for constituting the removal circuit.